Optical measurement systems are used for e.g. analysing properties or material contents of a target. The most common optical measurement devices are based on light absorbance/reflection of a target or fluorescence of a target. Such measurements are needed in laboratories and industry, for example. However, there is also a growing need for continuous monitoring of material contents in mass applications, i.e. in applications where a large number of devices are needed and low cost of the devices is important. One such application is monitoring the contents of sulphur in various processes, and especially monitoring contents of sulphur compounds and additives in fuel of a vehicle.
There are some disadvantages related to using prior art technology for monitoring material contents in mass applications, such as the monitoring contents of sulphur. The measurement of absorbance, reflectance and fluorescence are suitable only for certain materials. In measuring contents of sulphur compounds, for example, also the contents to be measured are very low, such as a few ppm. As a consequence, those methods do not provide adequate information on sulphur contents.
One method to solve this problem can be Raman spectroscopy. In Raman scattering phenomenon, upon collision with a molecule a photon loses some of its energy (Stokes radiation) or gains some energy (anti-Stokes radiation). In consequence, the radiation scattered from the molecules of material has a wavelength which is shifted from the wavelength of the initial radiation used for activation. The wavelengths of the scattered radiation are characteristic to a molecule, and they can be called Raman “signatures” of the molecule. For example, there are several signature bands in MIR (middle infrared) region representing sulphur bonds e.g. with coal. With Raman spectroscopy it is thus possible to get information on contents and types of molecules which include sulphur.
Prior art Raman spectrometers are suitable for laboratories, but there are some problems in using them in mass applications like the measurement of sulphur from fuel. Such Raman spectrometers are usually equipped with a high-power narrow-band laser source, a volume holographic grating for achieving high diffraction efficiency, heavily cooled CCD (Charge Coupled Device) camera or array, and a fibre-connected measurement probe with a beam splitter and filters. Such Raman spectrograph instruments provide simultaneous measurement of radiation within a large range of wavelengths and high spectral resolution, such as 4 cm−1. However, the instrument is very large-sized and expensive, and it is therefore not suitable for mass applications. Due to the low intensity of the Raman scattered signal and high spectral resolution of the spectrometer the measurement also tends to take a long time in order to achieve a sufficient signal-to-noise ratio.